Vertical xmas tree and workover assembly

ABSTRACT

A vertical subsea Xmas tree ( 200 ) having a production bore ( 203 ) and two valves ( 10 ) in the production bore ( 203 ). Each valve ( 10 ) has a valve gate ( 18, 20 ) which is functionally connected to an actuator ( 38, 40 ) moving the valve gate bidirectionally between an open position and a closed position. The production bore ( 203 ) is sealed off by the valves ( 10 ) in the closed position. The valve gate ( 18, 20 ) is provided with a string cutting edge ( 34 ).

The present invention relates to a vertical Xmas tree for a subsea well. In particular there is provided a novel Xmas tree which due to its features reduces the requirement of equipment during workover operations. A workover assembly and associated methods are also presented.

BACKGROUND

In the field of subsea wells a Xmas tree is used to control the flow of produced hydrocarbons from the well. It comprises valves adapted to open and close for such flow.

When performing workover operations within the subsea well, the operator needs access to the well through the Xmas tree. In such a scenario, the Xmas tree valves need to be in the open mode. Hence one needs additional measures to maintain well integrity. Typically, from a surface installation a riser is extended down to the well. In some cases a string, such as a coiled tubing or a wire, extends from the surface installation and into the well, through the riser and the Xmas tree to which the riser is connected.

On top of the well the Xmas tree is arranged (on the wellhead). Between the Xmas tree and the lower portion of the riser there are additional assemblies adapted for securing the well fluids. This includes a well control package (WCP) (also sometimes referred to as the lower riser package—LRP) arranged on top of the Xmas tree. The WCP contains two valves in the main bore which provide two barriers. These valves are either two gate valves, or one gate valve and one safety head. Both the gate valves and the safety head are able to cut a string extending through the valve / safety head. However the safety head exhibits better cutting capabilities than the gate valve.

To make it possible to rapidly disconnect the riser from the top of the subsea well, an emergency disconnect package (EDP) is arranged on top of the WCP. If a floating surface installation is used, it may drift off from the position directly above the well. The riser will then exhibit an inclination with the respect to the vertical line directly above the well. The EDP makes it possible to disconnect the riser from the WCP even if such an inclination is present. When disconnecting, the EDP will cut off a possible string extended through it and seal off the lower end of the riser in order to prevent the fluid in the riser to run out into the sea. The WCP will seal off the well and also cut off a possible string.

Thus, known Xmas trees are able to open and close for a fluid flowing through it. The WCP is able to close off the flow path and cut a possible string extending through it.

US patent publication U.S. Pat. No. 8,353,338 describes a well bore control valve which comprises a housing defining a through bore which is adapted to receive a string. The appending FIG. 1a , FIG. 1b , and FIG. 1c show such a valve. A first gate and a second gate are located within the housing. The gates are moveable in different directions transverse to the through bore between a through bore open position and a through bore closed position. By movement of the gates from the through bore open position to the through bore closed position, a string located between the gates can be sheared. A first seal seat forms a seal with one of the gates in the through bore closed position in order to seal the through bore.

A typical field of application of the valve described in U.S. Pat. No. 8,353,338 is a well bore. In the event of an emergency, a well bore control valve, as described in the said publication, is able to shear the string extending through the through bore. Such a string may for example be a workover/drilling conduit or intervention tooling. The valve will then close and seal the through bore, as a seal is formed between the first gate and the first seal seat.

Providing two gates provides an improved cutting performance of the string. One can however also imagine other types of cutting valves having only one gate, which may both exhibit sealing and cutting function.

An objective of the present invention is to simplify the equipment used when performing workover operations in a subsea well.

THE INVENTION

According to a first aspect of the present invention, there is provided a vertical subsea Xmas (VXT) tree having a production bore and two valves in the production bore. Each valve comprises a valve gate which is functionally connected to an actuator moving the valve gate bidirectionally between an open position and a closed position. The production bore is sealed off by the valves when they are in the closed position. According to the present invention, the valve gate is provided with a string cutting edge.

The valve actuators can be of any appropriate type, for instance comprising a hydraulic piston or a rotating stem. The string cutting edge of the valve gate is adapted to cut a string that extends through the valve. Such a string can typically be a coiled tubing or a wire that extends into a subsea well below the vertical subsea Xmas tree.

The vertical subsea Xmas tree can comprise a workover subsea control module that comprises an emergency disconnect package (EDP) control means and vertical Xmas tree control means.

The emergency disconnect control means in the workover subsea control module (WOSCM) makes the operator able to control an EDP which is landed above the VXT through the WOSCM. Thus, the WOSCM can typically be used to control valves of the VXT as well as the valves of an EDP landed on top of the VXT. Alternatively or in addition, the WOSCM can control other functions of the EDP, such as bleed off calves, annulus isolation valve and crossover valves.

According to a second aspect of the present invention there is provided a subsea workover assembly arranged above a subsea well. The subsea workover assembly comprises a vertical Xmas tree (VXT) having a production bore provided with a production valve and a subsea wellhead above which the vertical Xmas tree is landed. A riser extends downwards from a surface installation and an emergency disconnect package (EDP) is arranged between the riser and the vertical Xmas tree. Moreover, the emergency disconnect package has an emergency disconnect package through bore which is provided with an emergency disconnect package cut and seal valve. Characteristic to the second aspect of the present invention is that the production valve is a cut and seal valve.

Preferably, between the emergency disconnect package cut and seal valve and the production valve, there exists a valve-less path. In some embodiments this valve-less path may comprise a bore selector. However, a bore selector is not regarded as a valve herein. Alternatively one may state this feature in that the path between the emergency disconnect package cut and seal valve and the production valve is either valve-less or is having only one valve in the form of a bore selector. As will be appreciated by the skilled person, the feature reflects the fact that since the XT is provided with cut and seal valves, one does not need to arrange a cut and seal valve between the emergency disconnect package and the vertical Xmas tree.

Moreover, the emergency disconnect package can advantageously be landed directly onto the vertical Xmas tree.

The emergency disconnect package can comprise a bore selector and the vertical Xmas tree can comprise an annulus access bore. By operation of the bore selector, the operator can choose between access to the production or to the annulus bore of the VXT.

In an embodiment according to the second aspect of the invention, the vertical Xmas tree comprises a workover subsea control module that comprises an emergency disconnect package control means. The emergency disconnect package control means can for instance be adapted to control the emergency disconnect package cut and seal valve, and/or other functions in the emergency disconnect package.

In some embodiments, a jumper can be connected between the vertical Xmas tree and the emergency disconnect package in order to connect the workover subsea control module to the emergency disconnect package. Alternatively there may be provided a stab plate that provides communication between the workover subsea control module and the emergency disconnect package when the EDP is landed on the VXT.

According to a third aspect of the present invention, there is provided a method of running a vertical Xmas tree onto a subsea wellhead. The method includes the following steps:

-   -   a) arranging an emergency disconnect package above a vertical         Xmas tree in a vertical stack;     -   b) lowering the emergency disconnect package and the vertical         Xmas tree into a body of water and towards a subsea wellhead,         while attached to the lower end of a riser depending from a         surface installation;     -   c) landing and locking the vertical Xmas tree to the subsea         wellhead.

According to this aspect of the invention, step a) comprises establishing a valve-less path between an emergency disconnect package cut and seal valve in an emergency disconnect package through bore and a production bore in the vertical Xmas tree.

Step a) can advantageously comprise arranging a bore selector between the emergency disconnect package and the vertical Xmas tree.

Furthermore, step c) can comprise controlling a locking mechanism of the vertical Xmas tree by means of a workover subsea control module, when locking the vertical Xmas tree to the subsea wellhead.

Thus, a subsea vertical Xmas tree, a subsea workover assembly, and a method of running a subsea vertical Xmas tree are provided which result in simpler and less needed equipment than the prior art for performing workover operations in a subsea well.

EXAMPLE OF EMBODIMENT

While the invention has been described in general terms above, a more detailed and non-limiting example of embodiment is presented in the following with reference to the drawings, in which

FIG. 1a is a cross section view of a prior art gate valve in an open position;

FIG. 1b is a cross section view of the valve in FIG. 1a in a closed position;

FIG. 1c is an enlarged cross section view of the valve in FIG. 1 b;

FIG. 2 is a schematic overview of a floating surface installation performing workover operation in a subsea well;

FIG. 3 is a principle view of a workover setup according to prior art;

FIG. 4 is a principle view according to FIG. 3, however illustrating a setup according to the present invention; and

FIG. 5 is a principle view of a vertical Xmas tree in the process of landing on a subsea wellhead.

FIG. 1a to FIG. 1c illustrate a known valve 10 which is suited for well control of a subsea hydrocarbon well. FIG. 1a a shows the valve in an open position. Two gates 18, 20 are arranged axially on top of each other. They both have an aperture which in the open position are aligned with each other. To each gate 18, 20 there is connected a hydraulic actuator 38, 40. The actuators can pull the gates out of their open position shown in FIG. 1a and into their closed position, as illustrated in FIG. 1b . The actuators 38, 40 are also able to push the gates 18, 20 back into their open position. Hence, by controlling the actuators, the operator is able to open and close the valve 10.

At an edge portion of each aperture of the gates there is arranged a cutting element 34, 36 with a cutting edge. When the actuators 38, 40 pull the gates from the open position towards the closed position (FIG. 1b ), the cutting elements 34, 36 will engage a possible string that extends through the valve 10. Typically, such a string can be a wire or coiled tubing used for workover operation in a subsea well below the valve 10.

FIG. 1c shows portions of the gates 18, 20 in the closed position. This view also depicts seal seats 22, 26 arranged on axial opposite sides of the two gates.

The person skilled in the art will appreciate that other types than hydraulic actuators may be employed to move the gates back and forth, for instance an actuator which may operate by rotation of a rotating stem.

FIG. 2 schematically illustrates a setup according to the prior art during a workover operation on a subsea well. A floating installation 100 at the sea surface 101 is connected to a riser 103 that extends down towards a subsea well 105 below the sea floor 107. At the sea floor 107 a wellhead 109 is arranged at the upper portion of the subsea well 105. To the wellhead 109 a vertical Xmas tree 200 (VXT) is arranged.

To provide two barriers against well pressure in the well 105 during workover operation, a well control package 300 (WCP) is landed on top of the VXT 200. Moreover, an emergency disconnect package 400 (EDP) is arranged between the riser 103 and the WCP 300.

FIG. 3 is a schematic illustration of the setup according to prior art in FIG. 2. On top of the wellhead 109 (not shown in FIG. 3) are the VXT 200, the WCP 300, and the EDP 400 arranged. Shown above the EDP 400 is the riser 103 as well as an umbilical 111.

In this prior art setup, the VXT 200 comprises two production valves 201 in its main bore (production bore) 203. Both production valves 201 are able to open and close the production bore 203 in the VXT 200. In the VXT 200 there is also an annulus access bore 205 (annulus bore). The annulus bore 205 is provided with annulus bore valves 207 which also are adapted to open and close.

As discussed with reference to FIG. 2, above the VXT 200 is a well control package 300 (WCP). A WCP through bore 303 extends through the WCP 300. The WCP through bore 303 is provided with one cut and seal valve 301 and one safety head 302. The cut and seal valve 301 as well as the safety head 302 are able to cut a possible string being present in the through bore and to seal off the well. The safety head 302 is similar to a cutting gate valve.

Since the operator needs access both to the annulus bore 205 and the production bore 203 of the VXT 200, a bore selector 305 is arranged in the WCP 300. With the bore selector 305 the operator is able to select which of the bores, the production bore 203 or annulus access bore 205, he enters when lowering a tool down through the WCP through bore 303 of the WCP 300.

Still referring to the prior art solution depicted in FIG. 3, above the WCP 300 there is arranged an emergency disconnect package 400 (EDP). It has an EDP through bore 403 that aligns with the WCP through bore 303. The EDP 400 also has a cut and seal valve 401, which is able to cut a string extending through the EDP 400 and to retain the fluid in the riser 103 above the EDP 400. These functions take place when the operator needs to disconnect the EDP 400 from the WCP 300, typically during an excessive drift off situation, harsh weather conditions, or an approaching iceberg.

On top of the EDP 400 is shown the lower end of the riser 103 that constitutes a conduit from the EDP 400 to the floating installation 100. Also shown on top of the EDP 400 is the lower end of the umbilical 111 which may contain power supply, control cables and hydraulic channels from the floating installation 100 to the EDP 400.

For controlling components of the WCP 300 and the EDP 400, such as an EDP disconnection mechanism (not shown) and the cut and seal valves 301, 401, the EDP 400 also comprises a workover control module 409 (WOCM) (although in some embodiments such critical functions may be controlled directly by means of hydraulics). When the WCP 300 and EDP 400 have landed on the VXT 200 for workover operations, the WOCM 409 typically also controls the functions of the VXT 200, in addition to the functions of the EDP 400 and the WCP 300. The VXT 200 comprises a subsea control module 209, which is a component similar to the WOCM 409. The subsea control module (SCM) 209 arranged in the VXT 200 is used to control various components in the VXT 200 during normal production, i.e. when the EDP 400 and WCP 300 are not landed on the VXT 200.

FIG. 4 is a schematic view similar to the view of FIG. 3, however depicting an embodiment according to the present invention. In this embodiment, in the VXT 200 there are arranged cut and seal valves 10 in the production bore 203 and in the annulus access bore 205. Having such functionality in the bores of the VXT 200 makes it possible to omit the WCP 300, which is shown in the prior art setup of FIG. 3. The cut and seal valves 10 in the VXT 200 can for instance be of the type illustrated and discussed with reference to FIG. 1a to FIG. 1c . Thus, the cut and seal valves 10 of the VXT 200 according to the invention have cutting, sealing, opening and closing function.

Since the setup of this embodiment according to the invention is without the WCP 300, the bore selector 405 is now included in the EDP 400. Preferably the EDP 400 is landed directly onto the VXT 200.

Between the cut and seal valve 401 of the EDP 400 and the production bore 203 of the VXT 200 there is a valve-less path 250 when the EDP 400 is landed above the VXT 200. In particular, there are no valves present between the uppermost valve 10 in the production bore 203 of the VXT 200 and the lowermost valve in the EDP 400 (which is the EDP cut and seal valve 401). In this respect, the term valve-less shall mean that there are no valves present in the said path, except perhaps a possible bore selector which in some embodiments may be construed as a valve.

Through the valve-less path 250 there may be guided fluid and/or a string (not shown), such as a coiled tubing or a wire. A tool may also be attached to such a string and may enter through the valve-less path 250.

In a critical drift off situation during a workover operation, the cut and seal valves 10 in the VXT 200 cut a possible string (not shown) extended through the annulus bore 205 or the production bore 203 of the VXT 200, and seal off the bores 203, 205. The EDP 400 also cuts the possible string and seals off the EDP through bore 403 to retain fluid within the riser 103.

In the embodiment shown in FIG. 4, only one control module is shown. Whereas the setup according to prior art (cf. FIG. 3) included one SCM 209 and one WOCM 409, a setup according to the present invention may include a combined workover control module and subsea control module (WOSCM) 213. During normal production, the WOSCM 213 is adapted to control functions of the VXT 200, preferably in the same manner as an SCM of the prior art. During workover operations however, the WOSCM 213 also controls functions of the EDP 400, in addition to the functions of the VXT 200.

In a situation where the EDP 400 is landed onto the VXT 200, a jumper 215 can be connected between a wet-mate connection on the VXT 200 and a wet-mate connection on the EDP 400. Such a connection will typically be established by means of a remotely operated vehicle (ROV) (not shown). In this manner, the WOSCM 213 can be connected to the EDP 400. In a critical situation, where the EDP 400 needs to quickly disconnect from the VXT 200, the jumper can simply be snapped off. One can however also imagine an embodiment where the EDP 400 and the VXT 200 are provided with a stab plate (not shown) which provides communication between the WOSCM 213 and the EDP 400 when the EDP 400 is landed on the VXT 200. In such an embodiment the jumper 215 may be omitted.

In another situation the VXT 200 and the EDP 400 can be installed simultaneously on the riser 103. The jumper 215 can then be connected between the VXT 200 and the EDP 400 before they are lowered into the sea, unless they are provided with a stab plate as discussed above.

FIG. 5 schematically illustrates a situation where the VXT 200 is about to be landed on a subsea wellhead 109. The VXT 200 is arranged to the EDP 400 and they are both lowered on a riser 103. 

1. A vertical subsea Xmas tree having a production bore and two valves in the production bore, wherein each valve comprises a valve gate which is functionally connected to an actuator moving the valve gate bidirectionally between an open position and a closed position, wherein the production bore is sealed off by the valves in the closed position, wherein the valve gate is provided with a string cutting edge.
 2. The vertical subsea Xmas tree according to claim 1, comprising a workover subsea control module comprising emergency disconnect package control means and vertical Xmas tree control means.
 3. The vertical subsea Xmas tree according to claim 1, comprising an annulus access bore provided with two valves, which valves are gate valves having a valve gate provided with a string cutting edge.
 4. A subsea workover assembly arranged above a subsea well, the subsea workover assembly comprising a vertical Xmas tree having a production bore provided with a production valve; a subsea wellhead above which the vertical Xmas tree is landed; a riser extending downwards from a surface installation; an emergency disconnect package arranged between the riser and the vertical Xmas tree, wherein the emergency disconnect package has an emergency disconnect package through bore which is provided with an emergency disconnect package cut and seal valve; and wherein the production valve is a cut and seal valve.
 5. The subsea workover assembly according to claim 4, wherein between the emergency disconnect package cut and seal valve and the production valve there exists a valve-less path.
 6. The subsea workover assembly according to claim 4, wherein the emergency disconnect package is landed directly onto the vertical Xmas tree.
 7. The subsea workover assembly according to claim 4, wherein the emergency disconnect package comprises a bore selector and that the vertical Xmas tree comprises an annulus access bore.
 8. The subsea workover assembly according to claim 4, wherein the vertical Xmas tree comprises a workover subsea control module that comprises an emergency disconnect package control means.
 9. A method of running a vertical Xmas tree onto a subsea wellhead, the method comprising: a) arranging an emergency disconnect package above a vertical Xmas tree in a vertical stack; b) lowering the emergency disconnect package and the vertical Xmas tree into a body of water and towards a subsea wellhead, while attached to the lower end of a riser depending from a surface installation; c) landing and locking the vertical Xmas tree to the subsea wellhead; and wherein step a) comprises establishing a valve-less path between an emergency disconnect package cut and seal valve in an emergency disconnect package through bore and a production bore in the vertical Xmas tree.
 10. The method according to claim 9, wherein step a) comprises arranging a bore selector between the emergency disconnect package and the vertical Xmas tree.
 11. The method according to claim 9, wherein step c) comprises controlling a locking mechanism of the vertical Xmas tree via a workover subsea control module, when locking the vertical Xmas tree to the subsea wellhead. 